1. Field
This application relates generally to devices and methods to provide a set of sensory feedback information capabilities from robotic or prosthetic finger tips comparable to those provided by the human skin.
2. General Background and State of the Art
Present generations of robots lack most of the sensorial abilities of humans. This limitation prevents industrial robots from being used to carry on delicate tasks of enormous practical relevance (such as assembly operations) and, even more, it prevents the development of evoluted robots for off-factory jobs (agriculture, home, assistance to the disabled, etc). Future generations of robots may be increasingly featured by the massive use of dedicated sensors that will enhance substantially the limited ability of present robots to interact with the external world. Taction, vision and proximity are the sensory needs that, in combination or alone, are commonly accepted as desirable features of robots. Research on visual pattern recognition received considerable attention in recent years. Tactile recognition (the ability to recognize objects by manipulation) is an inherently active process. Unlike visual sensors (passive and located remotely from the object), tactile sensors must be put in contact with the object to be recognized and, even more, such contact should be competently organized in order to extract the maximum degree of information from manipulative acts.
Humans who have suffered amputations of their hands and arms are generally provided with prosthetic limbs. Increasingly these prosthetics incorporate electromechanical actuators to operate articulations similar to biological joints, particularly to control the fingers to grasp and hold objects. Recent research has revealed how arrays of biological tactile receptors distributed throughout the soft tissues of the finger tip are used normally by the nervous system to provide rapid adjustments of grip force. Due to limitations in currently available tactile sensing technology discussed below, currently available prosthetic fingers provide little or no sensing capabilities and cannot make use of these highly effective biological control strategies.
Tactile sensors are generally known and can be grouped into a number of different categories depending upon their constructions, the most common groups are piezoresistive, piezoelectric, capacitive and elastoresistive structures. The common feature of all of these devices is the transduction of local asperities (unevenness or a projection from a surface) into electrical signals. Tactile sensors are commonly used in the field of robotics and in particular with those robotic devices which pick up and place objects in accordance with programmed instructions; the so-called “pick and place” class of robot. Unfortunately, while it would be desirable for the above-listed groups of tactile sensors to respond in much the same way that the human finger does, many of them can provide only limited information about a contact with an object. This requires large numbers of separate structures or electrical characteristics that require extensive circuitry in order to obtain an output indicative of the surface which has been contacted. For robotics, the difficulties associated with their non-linear response mechanisms, their fragile structure, and the high cost of assembling many discrete components limits their use of the above groups in an industrial environment. There are difficulties with calibration, environmental survivability, and other factors which render them less than optimal for many applications in less restricted environments, particularly those associated with motor-actuated prosthetic hands and telerobotic systems intended to augment human performance.
The performance of prosthetic hands and robotic manipulators is severely limited by their having little or no tactile information compared to the human hand. A wide variety of technologies have been applied to solve the tactile sensing problem in robotics and medicine. Transduction mechanisms such as optics, capacitance, piezoresistance, ultrasound, conductive polymers, etc. have all yielded viable solutions but only for limited environments or applications. For example, most MEMS sensors provide good resolution and sensitivity, but lack the robustness for many applications outside the laboratory [1-3] (see text, infra, for citations to notes). Beebe et al. proposed piezoresistive silicon based MEMS sensor with a high tensile strength, but hysteresis and inability to sense shear force posed limitations [4]. Conductive particles suspended in elastomers can result in elastic materials whose resistivity changes with deformation. A recent enhancement of such materials called Quantum Tunneling Composites (QTC) greatly increases sensitivity and dynamic range but at the expense of mechanical hysteresis and simultaneous sensitivity to temperature and absorption of gases [5].
The curved, deformable nature of biological finger tips provides mechanical features that are important for the manipulation of the wide variety of objects encountered naturally. Multi-axis force sensing arrays have been fabricated using MEMS but they are not suitable for mounting on such surfaces or for use in environments that include heavy loads, dust, fluids, sharp edges and wide temperature swings [2, 3]. If skin-like elastic coverings are placed on top of sensor arrays, they generally desensitize the sensors and function as low pass temporal and spatial filters with respect to incident stimuli [6].